Elucidating the increased ohmic resistances in zero-gap alkaline water electrolysis

• Assumption of constant Tafel slope in I-V fitting overestimates the ohmic resistance. • In-situ ohmic resistance is up to 27% higher than ex-situ measured value. • Bubbles have a limited, yet significant contribution to ohmic resistance. • Bubbles can adhere to the diaphragm. • 4-terminal configuration is preferable in EIS and IV curve analysis. This study investigates the increased ohmic resistances observed in zero-gap alkaline water electrolyzers, aiming to provide insights that can help enhance electrolyzer efficiency and enable operation at higher current densities. Electrochemical impedance spectroscopy (EIS) has been employed in combination with chronopotentiometry, utilizing a custom-designed flow cell with nickel perforated electrodes and a Zirfon UTP 500 diaphragm. Observed differences in area-ohmic resistance values obtained through I-V fitting and EIS, are ascribed to a non-linear Tafel slope at higher current densities. Ohmic resistance values measured with EIS are up to 27% higher than the ex-situ determined value, a significantly smaller percentage than expected based on previous studies. The presence of bubbles outside and inside the diaphragm is identified as the key factor contributing to this increased resistance. We recommend the use of an improved fitting approach, accounting for non-linear Tafel behavior, and the use of a 4-terminal configuration when performing EIS measurements to minimize cable and contact resistance.